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VISITATION. 181 Consult Currier, Uislory of Ihc Religious Orders (New Vork, 189G). VIS MAJOR (Lilt., superior force). A term borrowed from the civil law and employed to denote an inevitable accident, that is, one which results from the operation of the forces o£ nature and which could not have been prevented bj' the exercise of care. See Carrier, Common ; Contract. VISSCHEB, vis'Kcr, Cornelis (c.1G29- (i-). A Dutch engraver. He was a native of Haarlem, and is thought to have been a pupil of P. Soutman, but little is known of his life. His portraits are. among the highest ex- am])les of Dutcli engraving. Between 1G4"J and ltil>2 he produced about 180 plates, including "The Ratcatcher," "The Pastry Cook," "Susanna at the Bath," and others after Italian and Low- land ina.sters. VIS'TULA (Polish TT'i.s/o, Ger. Weichsel). A river of Central Europe. It rises on the north slope of the Carpathian Mountains in Austrian Silesia, and flows first northeastward on the boundary between Galieia and Russian Poland, then across the latter in a north and north- west direction, and finally again northeastward through the Prussian Province of West Prussia, emptying into the Frisches Haff and the Baltic Sea through several arms, one of wdiich passes the city of Danzig (Map: Prussia, H 2). Its length is 650 miles. In its upper course it flows through a narrow valley with steep, wooded sides, but in Northern Poland the banks become low and the country more open until the river breaks through the Prussian ridge, where the banks are again high. In its lower course it di- vides repeatedly into parallel arms, inclosing wooded islands. The arms of the delta, one of which, the Nogat, enters the Frisches Haff through a large number of mouths, are subject to extensive change in their cliTinnels and in the volume of their discharge, and large sums are expended by the Prussian Government in works of regulation, partly to keep open a navigable channel, partly to protect the fertile delta region from inundation. The river is navigable for large river craft to the Austrian boundary, and for smaller vessels to Cracow. The traffic in grain, lumber, and other products is considerable, but navigation is rendered very difficult by con- stantly and rapidly shifting sandbanks. The chief cities on the Vistula are Cracow, Warsaw, and Danzig. Its chief tributaries are the San, the Bug, and the Pilica. A canal connects the river with the Oder. VISUAL SENSATION (Lat. visualis, relat- ing to sight, from lisiis, sight, from viderc, to see). Stimulation of the retina by ether waves gives rise to sensations of color and to colorless sensations (black, white, and gray — called sensa- tions of brightness quality by Hering). There are three chief characteristics of light sensa- tions — color-tone, brightness, saturation — w^hicli correspond respectively to three chief moments — wave-length, amplitude, purity — in the physical vibrations set up in the ether. (I) Color-tone (as red, orange, yellow, blue) corresponds in the first instance to wave-length. The longest wave (answering to red) has a length of about TOO ^/x, ("00 millionths of a millimeter) ; the shortest VISUAL SENSATION. (which corresponds to violet), a length of about 400 /xfi. Intermediate wave-lengths condition the color qualities orange, yclluw, green. Idue, etc. (2) llriyUtness corresponds to the ampli- tude of the ether wave. A stimulus of wide am- plitude gives rise to a sen.sation of great bright- ness, i.e. a brightness that approaches white. Black, which is as positive a sensation quality as red or while, is correlated with a stimulus of small amplitude or with the total absence of perijihcral stimulation (as in a completely dark- ened room). (3) >Suturalion depends upon the purity or homogeneity of the stimulus. A light of a single wave-length produces a sensation of a high degree of saturation; a light which includes nuuiy or all wave-lengths of the physical spec- trum, a sensation of a low degree of saturation (as a j'ellowish or reddish white) or one who.se saturation is zero (black, white, or gray). While this correlation of physical and psychical mo- ments is a general one, it has, nevertheless, sev- eral exceptions and modifications, the most im- portant of which are considered below. The colorless sensations (brightness qualities: Hering) form a one-dimensional .system running through the grays from light to dark, with wliite at one end of the series and black at the other. The series may be represented by a .straight line, at any point of which there is a brightness quality shading gradually on one side toward black, and on the other toward white. The place in the series of any given brightness is determined, theoretically, by the number of just noticeably dift'erent qualities between it and one end or other of the series, or practically by the position of the qualities which most resemble it on either side. The capacity for discriminating these qualities is known as .sensible discrimina- tion (q.v. ) for bi'ightness. It has been found that for moderate brightnesses the relative dif- ference limen, i.e. the ratio of a just noticeable difference to the absolute brightness value with which the observation is made, is practically constant at about -— - ; e.g. a gray of 100 photo- metric units is just noticeably different from one of 101 units. At the two extremes of the bright- ness series — the dead blacks and the brilliant whites — discriminability is much less. (See In- tensity OF Sensation.) It has been estimated, on the basis of experimental work, that there are almost 800 dift'erent brightnesses. The series of color-tones runs through the spectrum from red to violet and embraces also the purples. They may be conceived as l.ving around the periphery of a quadrilateral figure with red, yellow, green, blue at the four corners and the intermediates — oranges, yellow-greens, green-blues, purples, etc. — lying along the sides. A quadrilateral form is chosen instead of a circle because there are distinctive changes at the four points indicated — more or less abrupt changes in 'direction.' The figure also provides for the fact that the ends of the spectrum re- semble each other more than either end and the middle. Just noticeable differences have been worked out for color-tone as well as for brightness. Qualitative changes are much more rapid in some parts of the spectrum than in others. Sensible discrimination is greatest in yellow and blue- green and least in red and violet. In yellow and blue-green one can, under favorable conditions.